1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine and more specifically to an air-fuel ratio control system which utilizes the output of a dual oxygen concentration sensor arrangement to achieve feedback control of the fuel supply system.
2. Description of the Prior Art
The use of a so called three-way catalytic converter in an automotive exhaust system is well known. However, in order to achieve the simultaneous reduction of HC, CO and NOx, it is necessary to maintain the air-fuel mixture supplied to the combustion chamber or chambers of the engine at or very close to the stoichiometric air-fuel ratio (A/F) in order to maximize the conversion efficiency. The use of O2 sensors for this purpose is also widely known.
However, as the output characteristics Of O2 sensors vary from one sensor to another, a problem is encountered in that the unit to unit deviations in the sensors induce errors in the feedback control of the fuel supply whereby the stoichiometric air-fuel ratio is not maintained in the desired manner and the efficiency of the three-way conversion in the catalytic converter is inhibited.
To overcome this problem is has been proposed in JP-A-58-72647 to use two O2 sensors. One of the sensors is disposed in an exhaust conduit upstream of a 3-way catalytic converter while the other is disposed downstream thereof. The outputs of the two O2 sensors are fed to a control unit which in turn controls the amount of fuel injected by a fuel injector disposed in the engine induction system.
Similar arrangements are also disclosed in JP-A-1-113552 and U.S. Pat. No. 3,939,654 issued on Feb. 24, 1976 in the name of Creps.
An example of the control implemented in connection with this type of system is depicted in flow chart form in FIGS. 23 and 24. The routine depicted in FIG. 23 is such as to utilize the output VFO of the upstream O2 sensor to determine a feedback control factor .alpha. and is run at predetermined intervals (e.g. 4 ms) The first step of this routine is such as to determine if conditions (referred to as FRONT O2 F/B) which permit the use of the upstream side O2 sensor exist or not.
In the event that such conditions exist, for example: if the temperature of the engine coolant is not below a predetermined level of Tw; the engine is not being cranked/started; the engine has not just been started; the air-fuel mixture is not being deliberately enriched for engine warm-up; the output of the upstream O2 sensor has not yet switched from one level to another; or the engine is not undergoing a fuel cut, then it is deemed that conditions which enable the use of the sensor exist and the routine should flow to step S2. In this step the output OSR1 of the upstream O2 sensor is subject to A/D conversion, read and the value set in memory. In step S3 the instant value of OSR1 is compared with a slice level SLF (e.g. 0.45 volt) which is selected to represent, the air/fuel ratio. In the event that the outcome is such as to indicate that VFO.gtoreq.SLF (viz., lean) the routine goes to step S4 wherein a flag F1 is cleared (i.e. F1=0), while in the event that VFO&gt;SLF the routine proceeds to step S5 wherein flag F1 is set (F1=1).
As will be appreciated flag F1 is such as to indicate if the air-fuel mixture is richer or leaner than stoichiometric value. F1=0=lean, F1=1 rich.
In steps S6 to S8 the status of F1 for this run is compared with that of the previous one in manner to establish four possible paths for the routine to follow to one of steps S9 to S12. In these latter mentioned four steps an air/fuel ratio feedback correction factor .alpha. is subject to the following methods of derivation:
(i) In the case the routine flows from S6.fwdarw.S7.fwdarw.S9 the air-fuel ratio is indicated as just having undergone a rich lean change and .alpha. is derived by incrementing the instant value by a proportional component PL (.alpha.=.alpha.+PL). This tends to incrementally enrich the air/fuel mixture and thus shift the air-fuel ratio stepwisely back toward the stoichiometric value. PA1 (ii) In the case the routine follows a S6.fwdarw.S7.fwdarw.S10 path, the air-fuel mixture is indicated as just having undergone a lean.fwdarw.rich change. Accordingly .alpha. is derived by decrementing the instant value by a proportional component PR (.alpha.=.alpha.- PR). This tends to stepwisely lean the mixture back from the rich side. PA1 (iii) In the case of a S6.fwdarw.S8.fwdarw.S11 flow, a previously lean condition is again detected and the value of .alpha. is derived by adding an integrated component IL. This induces the A/F to return gradually toward the rich side. PA1 (iv) In the event of a S6.fwdarw.S8.fwdarw.S11 flow, a previously rich condition is again detected and the value of .alpha. is derived by subtracting an integrated component IR. This induces the A/F to return gradually toward the lean side.
The flow chart shown in FIG. 24 depicts a routine which utilizes the output of the downstream O2 sensor for deriving an .alpha. correction. This routine is run at predetermined intervals of 512 ms (for example). The reason for this relatively long delay between runs is to ensure that the feedback control which is primarily based on the output of the upstream O2 sensor (which is highly responsive to the changes in A/F) is not dulled by overly frequent application of the output of the downstream O2 sensor which, due to its position downstream of the catalytic converter, is more remote and much less responsive to changes in the air-fuel mixture being combusted in the combustion chamber(s) of the engine.
At steps S21-S25 the status of the downstream O2 sensor is checked to determine if the output (REAR O2 F/B) can be used for feedback control purposes. The output of the downstream O2 sensor is deemed to be unsuitable for feedback control correction when the conditions which effect the upstream sensor are found to be unsuitable; when the engine coolant temperature is found to be less than Tw (in this case 70.degree. C.) step S22; when the engine throttle opening LL is is fully opened (LL=1) - step S23; when the engine load/engine speed ratio Qa/Ne&lt;X1-step S24; or when in step S25 the downstream O2 sensor is found not to have been activated.
In the event that the appropriate requirements can be met, indicating that conditions wherein the output of the downstream 02 sensor can relied upon, the routine goes to step S26 wherein the output of the same VRO is A/D converted, read and set in memory. At step S27 the instant value of VRO is compared with a slice level SLR. In this instance the slice level is selected to represent the stoichiometric air-fuel ratio (e.g. 0.55 volt). In the event that it is found that the VRO.ltoreq.SLR the air-fuel mixture is deemed to be on the lean side and the routine flows to steps S28-S31. On the other hand, if VRO&lt;SLR the mixture is indicated as being on the rich side and the routine is directed to steps S32 to S35.
It should be noted that as the slice level SLR is set a little higher than SLF due to the fact that gases upstream and downstream of the catalytic converter are different and induce the sensors to exhibit slightly different output characteristics and to also allow for the different degradation rates between the two sensors.
At step S28 the PL value is incremented by a fixed value .DELTA.P (Viz., .DELTA.PL (PL=PL+.DELTA.PL). At step S29 the value of PR is decremented by a fixed value .DELTA.PR (PR=PR-.DELTA.APR. This has the effect of shifting the overall A/F in the rich direction.
At step S30 a constant value .DELTA.IL is subtracted from the integrated component IL in order to reduce the amplitude at which .alpha. increases as a result of the increase of PL in step S28. At step S31, a constant value .DELTA.IR is added to the integrated component IR in order to reduce the delay with which the output of the upstream O2 sensor switches from rich to lean, it being noted that this delay is induced by the increase in the PR value in step S29.
When the A/F is indicated by the output of the upstream O2 sensor to be on the lean side, .alpha. correction control is implemented in steps S28 to S31.
FIG. 25 shows a routine which is run at predetermined crankshaft rotation angle intervals (e.g. 30.degree. CA)and which is used to derive the fuel injection pulse width Ti [ms]. The first step S41 is such as to derive the basic injection pulse width Tp by table look-up using data which is recorded in terms of engine speed and the engine load. Following this in step S42, the sum of a plurality of correction factors (e.g. engine temperature related correction factor KTW) is calculated and at step S43 the actual injection pulse width Ti is derived using the equation: EQU Ti=Tp.times.Co.times..alpha.+Ts (1)
where Ts denotes the rise time of the fuel injector(s).
In step S44 the derived value of Ti is set in memory and used to produce the appropriate injection pulse(s).
However, the above type of air-fuel ratio control is such that the proportional component values (Pr, PL) used therein are obtained from mapped data wherein, in order to prevent engine surging, the data which falls in a predetermined special zone, is required to exhibits a particularly fine resolution (viz., changes in small increments). On the other hand, in order to render the adaptive or self-updating control more dynamic, the resolution of the data in updating zones is reduced.
In the event that, as shown in FIG. 15, the special zone (cross hatched) is smaller than one of the four updating zones A-D, a problem is encountered in that as the special zone is located in zone D, in the case the data is read out of the special zone on one or more runs of the control program(s) and then is read out of the zone D' (the zone surrounding the special zone) a sudden change in the correction amount is apt to to be induced and result in the deterioration of the emission control. To overcome this, it is possible to increase the resolution of the updating data, however, this requires a larger amount of ROM area.
A further problem is encountered in that, as the special zone overlaps part of the updating or adaptive zone, the updating zone is reduced and thus reduces the chances (frequency) of the updating process being carried out.